It is known to make packaging laminates by combining a cellulose-based, naturally decomposing basic layer with layers of biodegradable plastic which improve the tightness of the basic layer. This way total compostability of the material in the waste management of the packaging material can be achieved.
From the European application publication 534471, for example, a biodegradable laminate used as packaging material is known. This laminate consists of polyhydroxybutyrate-based copolymer, more precisely hydroxybutyric-hydroxyvaleric copolyester (HB/HV) coextruded together with a polyethylene film on a cellulose-based sheet. The polyethylene film is used as an auxiliary film in the coextrusion process, and after the process it is pulled of the laminate leaving the hydroxybutyric/hydroxyvaleric film attached to the sheet.
The biodegradable polymer in question has good barrier properties (barrier layer properties) in itself and is therefore a popular laminating agent, and it can be used to increase the tightness of cellulose-based basic materials noticeably.
In addition, the European application publication 603876 introduces the use of hydroxybutiric/hydroxyvaleric copolymer layers as water vapor barrier on both sides of an oxygen barrier layer of polyvinyl alcohol. The layers are laminated either on one side or both with a layer of cellulosic derivative or paper.
The multi layer structure in question does not provide satisfactory properties. Polyvinyl alcohol tends to dissolve in water which makes it difficult to be used under conditions of considerable humidity. Furthermore, the coextrusion of these materials is in practice difficult. And finally, polyvinyl alcohol is poorly compostible, in other words it is not biodegradable in the strict sense of the word.